1. Field of the Invention
This invention relates generally to an electric power steering apparatus for reducing a steering force of a driver by directly applying power of an electric motor to a steering system and, more particularly, to an electric power steering apparatus capable of informing a drive of a change in a road reaction force to thereby make the driver carry out proper steering.
2. Description of the Related Art
There has been known an electric power steering apparatus which comprises a control unit, a motor driver and a motor. In the apparatus, a target torque signal for driving the motor is generated by the control unit in correspondence with a steering torque generated by turning a steering wheel. A motor control signal for drive-controlling the motor based on the target torque signal is supplied to the motor driver composed of a bridge circuit; The motor is then PWM-driven via the motor driver to thereby apply an assist torque produced by the motor to a steering system.
The control unit drive-controls the motor by causing a signal corresponding to a motor current to be fed back (negative feedback) to the target torque signal to quickly equalize the motor current flowing through the motor.
Further, the target torque signal is corrected by a vehicle speed signal detected by a vehicle speed sensor, the target torque signal is reduced with an increase in the vehicle speed, a sufficiently large assist torque is added to the steering system when the vehicle speed is low and when the vehicle speed is high, a small assist torque is added to the steering system, thereby achieving reduction in the steering force of a driver when the vehicle speed is low and behavioral stability of the vehicle when the vehicle speed is high.
Further, there has been disclosed a conventional electric power steering apparatus in Japanese Patent Laid-Open Publication No. HEI-5-58318 in which when skidding of a vehicle is considerable, road information is transmitted to a driver via a steering wheel by magnifying a road reaction force from a road surface by reducing an assist torque relative to a steering torque.
In the electric power steering apparatus disclosed in Japanese Patent Laid-Open Publication No. HEI-5-58318, there are provided a vehicle speed sensor, a steering angle sensor and a lateral acceleration sensor. In the apparatus, a reference lateral acceleration G0 which is assumed to arise in a vehicle when the latter undergoes no disturbance is determined from a vehicle speed V detected by the vehicle speed sensor and a steering angle .theta. detected by the steering angle sensor. Determination is then made as to whether an absolute value .vertline.Gact-G0.vertline. which is a difference between an actual lateral acceleration Gact detected, as actually applied to the vehicle, by the lateral acceleration sensor and a reference lateral acceleration G0, is larger than a predetermined value "g". When it is larger (.vertline.Gact-G0.vertline.&gt;g), skidding (lateral slipping) of the vehicle is regarded to be large and an assist characteristic map for low road surface friction coefficient .mu. road which is previously set is selected. When it is small (.vertline.Gact-G0.vertline.&lt;g), skidding of the vehicle is regarded to be small, an assist characteristic map for high .mu. road set in advance is selected, and an assist amount is controlled in accordance with a steering force.
In the disclosed electric power steering apparatus, there is a tendency that the driver cannot feel accurately the behavior of the vehicle via the steering wheel as information since a subtle change in a road reaction force caused by the behavior of the vehicle is restrained owing to its arrangement in which an assist torque corresponding to the steering force of the driver is added to the steering system.
In recent years, such a tendency has been made significant by a tendency of reduction in a road reaction force resulting from reduction in the steering force and a tendency of making a steering gear ratio as small as possible.
With regard to such a steering feeling in respect of the low road reaction force, it is desired of a driver to carry out accurate steering operation for the behavior of the vehicle by accurately feeling the road reaction force in a critical region of the behavior of the vehicle or in operation at emergency.
For example, when a vehicle is likely to spin in a critical region of the behavior of the vehicle, a driver needs to swiftly carry out optimum steering operation by grasping the behavior of the vehicle.
It is the most readily available and effective method to become conscious of a change in a road reaction force associated with the behavior of the vehicle for a driver to grasp the vehicle behavior.
In the electric power steering apparatus disclosed in Japanese Patent Laid-Open Publication No. HEI-5-58318, the assist characteristic map for high .mu. road or low .mu. road is selected based on a difference between the actual, lateral acceleration Gact actually applied on the vehicle and the reference lateral acceleration G0 where the vehicle undergoes no disturbance. However, the reference lateral acceleration G0 is difficult to set since it varies with a road surface friction coefficient .mu..
Although a friction coefficient (.mu.) sensor may be mounted on a vehicle to set the reference lateral acceleration G0 in correspondence with the road friction coefficient .mu., the friction coefficient (.mu.) is difficult to detect accurately even when the friction coefficient (.mu.) sensor is used.
For example, the lateral acceleration G with a parameter of the friction coefficient (.mu.) in respect of a steering .theta. is provided with a linear characteristic when the steering angle .theta. falls in a predetermined range. However, when the steering angle .theta. exceeds the predetermined range, the characteristic becomes a nonlinear and the reference lateral acceleration G0 cannot be set.
Further, in the conventional electric power steering apparatus disclosed in JP Pat. Laid-Open Publication No. JP-HEI-58318, skidding of a vehicle is determined based on the absolute value .vertline.Gact-G0.vertline. of the difference between the actual lateral acceleration Gact and the reference lateral acceleration G0 and accordingly, although whether skidding of the vehicle is large or small can be determined, whether skidding, of the vehicle is caused by oversteering or understeering cannot be determined and the behavior of the vehicle cannot be accurately informed to a driver as the reaction force via a steering wheel.